U.S. patent number 6,166,740 [Application Number 08/228,050] was granted by the patent office on 2000-12-26 for method and system for viewing three-dimensional data for a tracked structure.
This patent grant is currently assigned to Hewlett Packard Company. Invention is credited to Thomas Malzbender.
United States Patent |
6,166,740 |
Malzbender |
December 26, 2000 |
Method and system for viewing three-dimensional data for a tracked
structure
Abstract
A method and system for viewing three-dimensional data which
corresponds to a structure that has been tracked throughout the
three-dimensional data is disclosed. Typically, structures which
require tracking so that they can be followed throughout
three-dimensional data are non-planar and, therefore, very
difficult to view. Initially, tracking data is obtained by tracking
a structure through the three-dimensional data. After the structure
has been tracked through the three-dimensional data, the structure
is displayed on a monitor or display device such that the portion
of the three-dimensional data which is pertinent to the tracked
structure is distinguishable from other data. As a result, a user
can readily and easily view the data associated with the tracked
structure. As an example, when the tracked structure is an artery,
the image of the tracked structure displayed on the monitor or
display device is not simply a planar slice of the
three-dimensional data, instead it is planar perpendicular to the
tracked structure and along a spline curve which follows the
tracked data points through the three-dimensional data.
Consequentially, for this example, the geometry of the displayed
image is a ruled spline cutting surface formed from the
three-dimensional data corresponding to the tracked structure.
Inventors: |
Malzbender; Thomas (Mountain
View, CA) |
Assignee: |
Hewlett Packard Company (Palo
Alto, CA)
|
Family
ID: |
22855562 |
Appl.
No.: |
08/228,050 |
Filed: |
April 15, 1994 |
Current U.S.
Class: |
345/419;
382/128 |
Current CPC
Class: |
G06T
17/00 (20130101) |
Current International
Class: |
G06T
17/00 (20060101); G06T 015/30 () |
Field of
Search: |
;395/118,119,120,127
;364/413.13-413.23 ;382/128 ;345/418,419,420,427 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
KR. Hoffman, Automated Three-Dimensional Vascular Reproduction from
Stereoangiograms, IEEE Engineering in Medicine & Biology
Society 10th Annual International Conference, 1988. .
K. Kitamura, Estimating the 3-D Skeletons and Transverse Areas of
Coronary Arteries from Biplane Angiograms, IEEE Transactions on
Medical Imaging, vol. 7, No. 3, Sep. 1988. .
N. Alperin, Automated Analysis of Coronary Lesions from
Cineangiograms Using Vessel Tracking and Iterative Deconvolution
Techniques, Computers in Cardiology, 1989. .
R.H. Selzer, Computer-Generated 3D Ultrasound Images of the Carotid
Artery, Computers in Cardiology, 1988. .
Ying Sun, Automated Identification of Vessel Contours in Coronary
Arteriograms by an Adaptive Tracking Algorithm, IEEE Transactionss
on Medical Imaging, vol. 1, No. 1, Mar. 1989. .
Chaudhuri et al., "Detection of Blood Vessels in Retinal Images
Using Two-Dimensional Matched Filters", IEEE Transactions on
Medical Imaging, vol. 8, No. 3, Sep. 1989, pp. 263-269. .
Hoffmann et al., "Automated Tracking and Computer Reproduction of
Vessels in DSA Images", Investigative Radiology, vol. 25, Oct.
1990, pp. 1069-1075 ..
|
Primary Examiner: Zimmerman; Mark K.
Claims
What is claimed is:
1. A method of displaying a structure that represents three-spatial
dimensions using a computer, a data base that includes an image
data set in which the structure is defined, and a display monitor,
comprising:
analyzing the image data set with the computer to determine a set
of medial axis points of the structure which extends through the
three-spatial dimensions;
extruding a display data set using the computer, the display set
being a subset of the image data set, by
defining an extrusion vector in the three spatial dimensions,
defining the display data set data to include data from the image
data set that lies within a set of vectors that are both (a)
parallel to the extrusion vector, and (b) that also pass through
one of the medial axis points;
wherein both of the image data set and the display data set include
data representing three-spatial dimensions; and
generating a display image from the display data set.
2. A method according to claim 1, wherein defining an extrusion
vector in the three spatial dimensions includes:
selecting a first point and a second point from the image data
set;
selecting an intermediate vector that extends from the first point
toward the second point; and
constraining the extrusion vector to lie within a plane that is
normal to the intermediate vector.
3. A method according to claim 2, wherein:
selecting the first point and the second point includes
selecting one of the medial axis points as the first point, and
selecting a different one of the medial axis points as the second
point; and
constraining the extrusion vector includes
constraining the extrusion vector to lie within a plane that is
normal to the vector extending from the first point toward the
second point.
4. A method according to claim 1, wherein analyzing the image data
set includes: analyzing the image data set using the computer to
select a first group of medial axis points and thereby track the
structure, each point in the first group separated by a
predetermined sampling interval from other points in the first
group.
5. A method according to claim 4, wherein analyzing the image data
set includes:
defining a second group of medial axis point to be all points lying
on a direct line between each adjacent pair of points in the first
group of medial axis points.
6. A method according to claim 1, wherein defining the display data
set further includes: defining the display data set to include all
data points found on any vector that is parallel to the extrusion
vector and also passes through a medial axis point.
7. A method according to claim 1, wherein generating a display
image includes:
using the computer to define coordinates of a viewpoint in the
three-spatial dimensions;
using the computer to select an image display that is chosen from
the perspective of the viewpoint.
8. A method according to claim 1, the method further using a user
interface device that is operatively coupled to the computer,
wherein defining the extrusion vector includes:
defining an intermediate vector that is a direct line between two
selective medial axis points; and
using the user interface device to select the extrusion vector from
the set of vectors that lie within a plane that is perpendicular to
the intermediate vector.
9. An apparatus for displaying a structure that represents
three-spatial dimensions, the structure being defined in an image
data set, comprising:
a computer driven to analyze the image data set and determine
therefrom a set of medial axis points of the structure which
extends through the three-spatial dimensions;
means for extruding a display data set which is a subset of the
image data set, by
defining an extrusion vector in the three spatial dimensions,
defining the display data set data to include data from the image
data set that lies within a set of vectors that are both (a)
parallel to the extrusion vector, and (b) that also pass through
one of the medial axis points;
wherein both of the image data set and the display data set include
data representing three-spatial dimensions; and
a display that is operatively coupled to the computer to display a
display image from the display data set.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to techniques for viewing
three-dimensional data and, more particularly, to a method and
apparatus for viewing data for a structure which has been tracked
through three-dimensional data.
2. Description of the Related Art
In using three-dimensional data, images of many structures do not
remain in a given two-dimensional plane. As a result, when viewing
three-dimensional data on a conventional computer screen, it is
difficult to identify and trace these non-planar structures within
the three-dimensional data.
Tracking approaches are known, but these known tracking approaches
operate only on two-dimensional data. See, Hoffmann et al.,
Automated Tracking and Computer Reproduction of Vessels in DSA
Images, Investigative Radiology, vol. 25, pp. 1069-75, October,
1990; and Chaudhuri et al., Detection of Blood Vessels in Retinal
Images Using Two-Dimensional Matched Filters, IEEE Transactions on
Medical Imaging, Vol. 8, No. 3, pp. 263-269, September 1989. Even
assuming the known tracking approaches track the structure
correctly in two-dimensional data, the images produced from
two-dimensional data are inferior to those images that would be
produced by three-dimensional data.
In any case, once a structure is tracked, it must be displayed for
a user. Conventionally, a tracked structure could at best be merely
hightlighted in the original dataset. In order to visualize the
structure, the user would have to manually search through the
three-dimensional data until the structure was found and then, in
order to follow the structure throughout the data, the user would
have to make manual adjustments to change to different planar data
slices. The conventional display methods make it difficult for a
user to extract or interpret the data concerning the tracked
structure. The conventional methods also make no effort to minimize
the amount of data displayed which does not pertain to the tracked
structure.
Three-dimensional data offers significantly more information to a
user than does two-dimensional data. This additional dimension
makes the images eventually displayed more meaningful to the user.
One type of three-dimensional data is magnetic resonance imaging
(MRI) data which is used by doctors for diagnosis of patients. MRI
data is becoming useful in the diagnosis and treatment of blockages
existing in a patient's coronary arteries.
Consider for example the problem of tracking or viewing arteries.
Currently, a medical doctor's diagnosis is made by viewing only a
single (two-dimensional) slice of MRI data at a time. Although the
particular slice of data is accurate, it is difficult for the
doctor to accurately evaluate the health of the coronary arteries
because only discrete slices of data can be viewed one at a time.
Although a doctor can switch back and forth between slices in an
attempt to follow an artery, such data is confusing and difficult
to interpret because there is no information as to what happens to
the artery between such data slices. Also, due to the interplay of
various images, locating the artery being evaluated in each slice
of data is difficult. Hence, the doctor's diagnosis is hindered
because three-dimensional tracking is unavailable to assist the
doctor by identifying and/or extracting the relevant data
concerning the structure being tracked.
Thus, since known methods for displaying tracked structures are
inadequate, there is a need for a technique to visualize
structures, such as arteries, which have been tracked through
three-dimensional data so that the artery or other tracked
structure can be easily and readily viewed.
SUMMARY OF THE INVENTION
Broadly speaking, the invention relates to a technique for
displaying a non-planar structure contained in and tracked through
three-dimensional data such that the displayed data follows the
non-planar nature of the tracked structure. The invention can, for
example, be implemented as a method or as a system.
As a system, the invention includes a device for receiving tracking
data points (within the three-dimensional data) for the structure
to be displayed, a controller for sampling the three-dimensional
data in a planar direction between the tracking data points for the
structure and along a spline curve formed by the tracking data
points, and a display device for displaying the sampled data for
the structure.
As a method, the invention receives tracking data (e.g., medial
axis points) in the three-dimensional data for the structure. Next,
the method samples, in a planar direction, the three-dimensional
data along a spline curve formed by the tracking data. Thereafter,
the sampled data is displayed on a monitor or other display
device.
In the case where the structure tracked is an artery, the tracking
data is preferably medial axis points of the artery, and the
three-dimensional data is preferably MRI data. The resulting image
of the artery is a length-wise cross-section which has a
ribbon-like appearance.
The invention offers numerous advantages as compared with the prior
art. Namely, the invention provides a visualization technique that
is suitable for visualizing structures which have been tracked in
three-dimensional data. The visualization technique enables tracked
structures to be easily and readily viewed on a computer display or
monitor.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be readily understood by the following detailed
description in conjunction with the accompanying drawings, wherein
like reference numerals designate like structural elements, and in
which:
FIG. 1 is a block diagram of a tracking system according to the
invention;
FIG. 2 is a flow chart of a basic embodiment of a method according
the invention;
FIG. 3 is a flow chart of a first embodiment of a method according
to the invention;
FIG. 4 is a schematic diagram of a first embodiment of a spatial
filter;
FIG. 5A is a perspective view of a second embodiment of a spatial
filter;
FIG. 5B is a side view of the spatial filter illustrated in FIG.
5A;
FIG. 6 is a flow chart of a basic embodiment of a tracking method;
and
FIG. 7 illustrates an example of a ruled spline surface (slicing
ribbon) produced by the invention for a coronary artery within MRI
data.
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the invention are discussed below with reference to
FIGS. 1-7. However, those skilled in the art will readily
appreciate that the detailed description given herein with respect
to these figures is for explanatory purposes as the invention
extends beyond these limited embodiments.
Once a structure is tracked throughout three-dimensional data, the
tracked structure must be displayed so that the structure can be
easily viewed by a user. Typically, structures which require
tracking so that they can be followed throughout three-dimensional
data are non-planar and, therefore, very difficult to view.
Conventional approaches to tracking and displaying, which simply
display a planar slice of the data, are inadequate. This invention
provides a novel way to display the tracked structure so that the
data associated with the structure can be easily and readily
observed.
Typically, tracking techniques produce data points (tracking data)
which are associated with the structure being tracked. The tracking
data may, for example, be boundary points or medial axis points of
the structure as it progresses through the three-dimensional data.
The data points are normally, though not necessarily, stored in a
sequential list. The preferred tracking technique is described in
detail below.
The invention provides a novel method and system for viewing
three-dimensional data which corresponds to a structure that has
been tracked. By making use of such tracking data, the invention is
able to display on a computer display or monitor only that portion
of the three-dimensional data that is pertinent to the tracked
structure. As a result, a user can readily and easily view the
tracked structure.
The image of the tracked structure displayed on the computer
display or monitor is not simply a planar slice of the
three-dimensional data, instead it is planar perpendicular to the
tracked structure and along a spline curve which follows the
tracked data points (e.g., medial axis points) through the
three-dimensional data. Consequentially, the geometry of the
displayed image is a ruled spline cutting surface formed from the
three-dimensional data corresponding to the tracked structure.
FIG. 1 is a block diagram of a system for displaying
three-dimensional data according to the invention. The system 2
includes a computer 4 which controls operation of the system 2. The
computer 4 can be a computer, a microprocessor or other circuitry.
The computer 4 is connected to a program memory 6 which stores a
visualization program to be executed by the computer 4. The
operation of the visualization program is described in detail below
with respect to FIGS. 2 and 3. The computer 4 is also connected to
a data memory 8. The data memory 8 stores the three-dimensional
data (the dataset) as well as sampled data produced by the
visualization program. The computer 4 is further connected to a
display 10 and an orientation control device 12. The display 10
displays the data corresponding to the tracked structure which has
been identified by the visualization program. The orientation
control device 12 can be used to select an orientation from which
to view the data associated with the structure.
FIG. 2 illustrates a basic embodiment of the visualization method
according to the invention. The visualization method 16 is carried
out when the visualization program is executed. The method 16
initially obtains 18 tracking data for the tracked structure. The
three-dimensional tracking data is produced by a tracking program
which could also reside in the program memory 6 of the system 2.
The operation of a preferred tracking program is described below
with reference to FIG. 6.
Next, the three-dimensional data is sampled 20 in a planar
direction and along a spline curve formed based on the tracking
data. Although the planar direction in which the three-dimensional
data is preferably sampled remains constant, the sampling position
will follow the tracked structure in the three-dimensional data
because the sampling 20 is performed along the spline curve
corresponding thereto. As a result, the sampling 20 (although
uniformly in the planar direction) will sample the data
corresponding to the non-planar structure in the plane in which the
structure is in at each sample point.
Finally, the sampled data is then displayed 22 on the computer
display or monitor. The displaying of the sampled data illustrates
the data along the spline curve which is associated with the
tracked structure. Specifically, the sampled data produced by the
visualization program is displayed on the computer display or
monitor as a ruled splined cutting surface. As a result, the user
can view the cross-section of the structure in the planar direction
in a single view because the data displayed follows the
structure.
FIG. 3 is a flow chart of a first embodiment of the visualization
method according to the invention. In this embodiment, the method
24 begins by obtaining 26 a sequential list of medial axis points
for a non-planar structure within the three-dimensional data. Here,
although medial axis points are used, other tracking data could
also be used. However, for tubular structures such as arteries, a
sequential list of medial axis points is preferred.
Next, a spline curve is formed 28 is formed from the medial axis
points. When the medial axis points are arranged in a sequential
list, they can be linked together to form the spline curve. A
second-order spline curve would require only linear interpolation
between the medial axis points. For higher order spline curves more
sophisticated interpolation would be used, with the result being a
smoother spline curve.
An end-to-end direction for the structure is also determined 30.
The end-to-end direction can be determined in a number of ways. One
way is to set the end-to-end direction to the direction of a line
which would connect the first and last medial axis point in the
sequential list. The end-to-end direction merely provides the
general direction for the structure in the three-dimensional data.
Thereafter, a planar direction, which is perpendicular to the
end-to-end direction, can be selected 32.
The sampling 34 of the three-dimensional data then occurs. The
sampling 34 is performed in the planar direction and between the
medial axis points and along the spline curve. By sampling 34 in
the planar direction, the sampled data should contain the
length-wise cross-section of the data for the structure. Finally,
the sampled data is displayed 36.
Although the use of an end-to-end direction simplifies the sampling
and displaying of the data corresponding to the structure, the use
of an end-to-end direction can be eliminated and the sampling of
the data could always be performed in the direction perpendicular
to the spline curved formed from the medial axis points. In either
case, the sampling is along the spline curve.
The visualization method may also include a step in which a viewing
direction is changed. The viewing direction is a point (relative to
the tracked structure) from which the user views the sampled data.
The viewing direction can be altered by the user via the
orientation control device 12. The displayed images are
correspondingly changed in accordance with the new viewing
direction using known computer graphic techniques (e.g.,
transformation martix).
In a preferred implementation of the invention, the structure being
tracked is an artery and the three-dimensional data is discrete MRI
data. The preferred use of the invention is then to view arteries
which have been tracked throughout MRI data. In this case, it is
preferable that the sampling 34 of the three-dimensional data be
performed a predetermined distance above and below the medial
access points. Because the data corresponding to the artery is
displayed in a ruled spline surface the user can visualize the
entire cross-section of the artery without having to manually
adjust the data slice being viewed.
FIG. 7 illustrates an example of such a ruled spline surface
produced by the invention for a coronary artery in MRI data. The
resulting image of the artery is a length-wise cross-section which
has a ribbon-like appearance.
Displaying of the artery requires interpolation of the
three-dimensional data because the data is typically discrete
points which are stored in an array or matrix. The interpolation is
trilinearly performed on the discrete data to produce data suitable
for conventional graphics hardware. The graphic hardware will
perform Gouraud shading so that the displayed images will appear as
though the data were continuous.
In the preferred embodiment, the tracking data is a sequential list
of medial access points for the structure in the three-dimensional
data. Typically, this sequential list of medial access points would
have been produced by a tracking technique. A preferred tracking
technique is described below with reference to FIGS. 4-6.
The tracking technique which tracks a structure in
three-dimensional data. The tracking technique uses a geometric
shape to model the structure to be tracked. Tracking is achieved by
adapting the geometric shape at a given three-dimensional point
within the structure being tracked until the geometric shape best
fits the structure being tracked. Using this "best fit" information
a subsequent point within the structure being tracked can be
identified. The geometric shape is then adapted at or near the
subsequent point until a best fit is identified. Note, the
geometric shape used for tracking does not necessarily have the
same shape as that of the structure being tracked.
The three-dimensional data can be of a variety of types. For
example, the data type may be MRI data. It should be noted that
typically MRI data is discrete data which forms a matrix-like
volume in three-dimensions. The invention, however, preferably and
more precisely operates on continuous data by interpolating from
the matrix points of the discrete MRI data.
The "best fit" information is obtained from a digital filter and a
transformation matrix. The digital filter is configured in the
geometric shape which models the structure being tracked.
Transformation matrixes are a conventional computer graphics
technique. See, Foley and Van Dam, Fundamentals of Interactive
Computer Graphics, Section 7.5, 1982.
The "best fit" information is obtained for a given
three-dimensional point within the structure being tracked. The
"best fit" is determined using filter responses which are an
indication of whether the digital filter (as transformed by the
transformation matrix) is properly aligned with the structure being
tracked. Various responses are obtained for different alignments.
Based on the responses, the invention determines the orientation of
the structure at the given three-dimensional point within the
three-dimensional data. In effect, the orientation chosen is that
which corresponds to the greatest response (i.e., "best-fit"
information). A next point within the structure is thereafter
determined using the previous point as well as the orientation of
the structure at the previous point. After obtaining orientations
of the structure at various adjacent but separated points in the
three-dimensional data, the points can be visually linked together
or otherwise distinguishably displayed so that the user can easily
follow the structure through the three-dimensional data.
FIG. 4 is a schematic diagram of a spatial filter 38. The spatial
filter 38 is a two or three dimensional digital filter. The spatial
filter 38 includes a first ring 40 of radius r1 and a second ring
42 of radius r2. The first ring 40 is illustrated as having seven
filter coefficients 44 (each coefficient being represented by a
small circle), and the second ring 42 is illustrated as having
seven filter coefficients 46 (each coefficient being represented by
a small square). The first ring 40 and the second ring 42 are
coplanar. Although FIG. 4 illustrates the filter coefficients 44,
46 as having a small area or volume, the filter coefficients 44, 46
actually operate at points, which are referred to as sample points.
Moreover, the number of filter coefficients 44, 46 can vary
depending on the accuracy desired and the processing time
available.
Although the rings 40 and 42 of the spatial filter 38 illustrated
in FIG. 4 are circular rings, other shapes may be used. Ultimately,
the geometric shape in which the coefficients are arranged should
model the structure being tracked.
The following discussion will concentrate on the spatial filter
illustrated in FIGS. 4 and 5A because an important application or
use of the invention is to track arteries which have a tubular
structure. In which case, the spatial filter preferably has a
geometric shape which is circular or tubular so as to model
arteries.
FIGS. 5A and 5B are views of a spatial filter 48 which is
preferable for tracking arteries. The spatial filter 48 is
preferable for searching for arteries because the filter itself has
a tubular structure thus making it more computationally efficient
and accurate in tracking arteries. The spatial filter 48 includes a
first tier 50 which includes an inner ring 52 and an outer ring 54.
These rings 52, 54 are constructed as are the rings 40, 42 shown in
FIG. 4. That is, each ring 52, 54 consists of filter coefficients
44, 46 arranged in a circular ring. The spatial filter 48 also
includes a second tier 56 which includes similar rings 58, 60. The
first tier 50 is separated from the second tier 56 by a distance d.
The radii r are preferably 0<r<1 with the inner ring having a
radius r and the outer ring having a radius of 1/r. The actual
radius of the rings of the filter are scaled to the desired size in
accordance with this relationship.
Preferably, when tracking arteries the filter coefficients 44, 46
have a value of one and have opposite signs. For example, with a
bright blood MRI data, the filter coefficients 44 would be +1 and
the filter coefficients 46 would be -1. The response R of the
spatial filter 48 at the three-dimensional point (x,y,z) as
oriented as shown in FIG. 5A is computed by the following equation.
##EQU1## where .delta. is a delta function, which takes on a value
of 1.0 if its argument is zero, otherwise it takes on a value of
zero; .gamma. is the radius of the inner ring; .gamma..sup.-1 is
the radius of the outer ring; and M is the number of coefficients
in a tier. The equation for the case where there are two tiers.
FIG. 6 is flow chart of a basic embodiment of a tracking method
according to the invention. The tracking method can be carried out
by a tracking program stored in the program memory 6 and executed
by the computer 4. The tracking method 62 illustrated in FIG. 6
includes a number of operations.
Initially, a dataset of three-dimensional data is obtained 64. The
dataset can be held in a data memory 8. Next, a structure to be
tracked is identified 66. For example, in accordance with FIG. 1, a
user would use the pointing device 14 to indicate on the display 34
a particular structure within the three-dimensional data which the
user desires to track through the dataset. As an example, the
structure identified 66 might be a coronary artery.
Once the structure is identified 66, a first medial axis point for
the structure is approximated 68. For an artery, the first medial
axis point could be selected by a user via the pointing device 14
to roughly approximate 68 the center of the artery. In this case,
the identifying 66 and the approximating 68 could be performed by a
single user action. Alternatively, the technique could be enhanced
by having the tracking program approximate 68 the first medial axis
point for the structure.
Once the first medial axis point has been approximated 68, an
orientation and scale of the structure at the medial axis point can
be approximated 70 using a predetermined geometric shape. This
approximation 70 is carried out by the computer 4 as it executes
the tracking program. The tracking program includes executable code
to implement the spatial filter and a transformation matrix.
The spatial filtering performed by the tracking program performs
the spatial (digital) filtering using a predetermined geometric
shape. For example, for tracking arteries, the predetermined
geometric shape would be circular or tubular (see spatial filter
38, 48).
The transformation matrix implemented by the tracking program
varies three-degrees of freedom of the predetermined geometric
shape at a three-dimensional location within the structure so that
a maximized response can be identified. In effect, the
transformation matrix operates to rotate the spatial filter about
the three-dimensional location. At each of many discrete different
orientations, the filter response is measured. Once filter
responses for a predetermined number of orientations have been
obtained, the tracking program selects the orientation of the
structure being tracked to be the orientation of the spatial filter
yielding the maximized response.
The transformation operates to approximate three-degrees of freedom
of the spatial filter at a three-dimensional location. In total,
the tracking program controls six degrees of freedom. The
three-dimensional location within the structure represents three
dimensions (e.g., the origin of the x',y',z' coordinate system) and
the other three degrees of freedom (determined by the
transformation means) are the two angles in spherical coordinates,
theta (.theta.)and phi (.phi.), and a scale (.kappa.).
The spherical coordinates specify the orientation of the structure
as estimated by the predetermined geometric shape. In a circular or
tubular geometric shape, the scale corresponds to radius. Thus,
once the maximized response is identified, the tracking program
determines or approximates the radius and orientation of the
structure at the first medial axis point. Of course, if the scale
is uniform in the structure being tracked, the scale need not be
varied in searching for the maximized response.
Given that the spatial filter must be placed at the
three-dimensional location, the spatial filter must be translated
from the base coordinate system (x,y,z) to the coordinate system
having as its origin the three-dimensional location (x',y',z'). The
tracking program implements this translation operation with a
translation matrix. However, it is preferred that the
transformation matrix incorporate both the translation and the
rotation (orientation) operations. The preferred combined matrix is
then as follows. ##EQU2##
As discussed above, the three-dimensional data can be MRI data.
Typical MRI data has twelve (12) bit values for its intensity
distributions. Processing can be simplified by normalizing the MRI
data to smaller data values. For example, by normalizing the MRI
data to values between 0 and 255, only eight bit values would be
necessary; however, some loss of resolution would result. Further,
when the dataset is a bright blood MRI dataset and the filter
coefficients 8, 10 are +1 and -1, respectively, then the response
obtained (from spatial filter and transformation matrix) is
preferably computed as follows. The response is the sum of the
products of the filter coefficients 44, 46 and the intensity values
of the data at the sample points (i.e., the three-dimensional
locations of the filter coefficients). As mentioned, the intensity
values at the sample points represent those of continuous data
because interpolation (from nearest matrix points) is used to
accurately estimate the actual value. As a result, when the scale
and orientation of the spatial filter 38, 48 are properly aligned
with the structure being tracked, the response will be maximized
because the additive value from the inner ring 40, 52 and 58 will
be large and the subtractive value of the outer ring will be
small.
In general, the tracking program operates to check numerous radius
sizes and numerous orientations and then selects the radius and
corresponding orientation which yields the maximized filter
response. When the response is maximized, the scale represents the
radius of the wall because the actual artery wall would be between
the inner ring and the outer ring.
In any case, returning to FIG. 6, once the orientation and scale of
the structure at the medial axis point are approximated 70, it is
necessary to track the structure to a subsequent position.
Specifically, a next medial axis point for the structure is
approximated 72 based on the previous medial axis point and the
orientation.
The next medial axis point can be determined or approximated as
follows. First, by projecting out from the medial axis point a
predetermined distance in a direction along the medial axis, a
projected axis point is found 72. One way this can be achieved is
by projecting the predetermined distance out from the medial axis
point in the direction which is perpendicular to the plane in which
the orientation resides. Well known vector approaches can be used
to make the projection. Second, the next medial axis point for the
structure being tracked can then be approximated 74 based on the
projected axis point. For example, using the projected axis point
as a rough approximation of the position of the position of the
next medial axis point, a constrained search can be performed in
the near vicinity of the projected axis point. The point which
yields the maximized filter response is chosen as the next medial
axis point. This searching would involve checking the orientation
and radius of various points in the near vicinity of the projected
axis point. Using this approach, the maximized filter response then
yields not only the next medial axis point, but also the scale and
orientation for the next medial axis point, thereby combining the
operations of block 70 with those of block 74.
Blocks 70 through 74 thereafter repeat for each subsequent axis
point for the structure until a decision block 76 determines that
the structure has been fully tracked. Once fully tracked, the
tracking of the structure is completed and the information produced
by the tracking may be used in any way desired. Typically, the
tracked structure will be displayed in the display 10 of the
tracking apparatus 2 to facilitate doctor's diagnosis.
Additional details on the preferred tracking technique are
described in U.S. application Ser. No. 08/228,042, entitled "Method
and Apparatus for Tracking Structures in Three-Dimensional Data",
filed concurrently herewith, and hereby incorporated by
reference.
The many features and advantages of the invention are apparent from
the written description and thus it is intended by the appended
claims to cover all such features and advantages of the invention.
Further, since numerous modifications and changes will readily
occur to those skilled in the art, it is not desired to limit the
invention to the exact construction and operation as illustrated
and described. Hence, all suitable modifications and equivalents
may be resorted to as falling within the scope of the
invention.
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